The present invention relates to a transfer-and-fixation system for a liquid-development electrophotographic apparatus for transferring a toner image from an intermediate transfer roller onto a printing medium and fixing the transferred toner image on the printing medium, by use of a backup roller.
In a liquid-development electrophotographic apparatus, a melt transfer system for fixing a toner image on a printing medium is desirably performed such that, when toner particles are to be brought into contact with the printing medium for transfer onto the medium, the toner particles and the medium have a temperature not lower than the melting temperature of toner particles. In the course of transfer, a backup force is applied to the back side of the medium so as to establish close contact between the toner particles and the medium, whereby the molten toner particles are transferred onto the medium by means of adhesion thereof.
Conventionally, as shown in FIG. 24, in a melt transfer-and-fixation system where toner is melted, and the molten toner is transferred onto and fixed on paper by means of adhesion thereof, in order to increase transfer efficiency and fixation strength, the temperature of a transfer roller and that of a backup roller must be set sufficiently high (e.g., 150xc2x0 C.) in relation to the melting temperature of toner.
Heating an intermediate transfer belt, which has good releasability (low surface energy), to high temperature, as shown in FIG. 3, causes toner cohesion to drop greatly, so that the difference between toner cohesion and the surface energy of the intermediate transfer belt becomes small; as a result, surface tension thins toner image.
Further, before a toner image is transferred onto the intermediate transfer belt, the intermediate transfer belt must be cooled in order to protect members which come into contact with the intermediate transfer belt (e.g., a photosensitive drum) from heat and to prevent defective transfer which would otherwise result from melting of toner. In order to cope with such problems, conventionally, the intermediate transfer belt is cooled by use of a cooling unit such as a cooling fan, and a thin intermediate transfer belt has been employed for reducing the thermal capacity thereof.
However, in view of strength retention and other factors, the thickness of the belt can be reduced at most to about 50 xcexcm. Therefore, the thermal capacity of the belt cannot be sufficiently minimized, thereby causing substantial amount of energy to be consumed for cooling.
FIG. 25 shows a known structure of an intermediate transfer body (disclosed in Japanese Patent Application Laid-Open (kokai) No. 2000-56575). The intermediate transfer body assumes the form of a roller and includes a rigid drum which serves as a core thereof and is made of metal such as aluminum. The drum is electrically conductive so as to allow application thereto of voltage from, for example, a shaft thereof for electrostatically transferring a toner image from a photosensitive body onto the intermediate transfer body. Also, the drum has hardness suited for application of a pressure required for melt-transferring toner particles, which have been transferred onto the intermediate transfer roller, onto medium such as paper. On the drum, an elastic body layer which is electrically conductive and resistant to heat is formed. On the elastic body layer, a high-stiffness surface layer which is electrically conductive and resistant to heat and has appropriate releasability and preferably resistance to silicone oil is formed.
The high-stiffness surface layer is, for example, a heat resistant, electrically conductive polyimide film having a thickness of about 10-50 xcexcm coated with fluorosilicone rubber and functions to reduce expansion and contraction of the intermediate transfer body.
However, high-stiffness materials (e.g., polyimide) which has been conventionally used for a surface layer of an intermediate transfer body in a color electrophotographic apparatus are expensive.
The present invention has been accomplished in view of the foregoing, and an object of the invention is to ensure high transfer efficiency through enhancement of toner cohesion and toner adhesion to paper, while maintaining members (such as a photosensitive drum) which come into contact with an intermediate transfer roller at a temperature not higher than the withstand temperatures of the members, to thereby eliminate the need to cool the members for protection from heat.
Another object of the present invention is to carry out printing with high image quality by maintaining toner cohesion on the intermediate transfer roller having good releasability at a sufficiently high level as compared with surface energy of the intermediate transfer roller, to thereby avoid thinning an image.
Yet another object of the present invention is to provide an inexpensive intermediate transfer body layer structure with high stiffness that is suitably applicable to an intermediate transfer roller without the use of expensive surface layer material.
In a transfer-and-fixation system for a liquid-development electrophotographic apparatus of the present invention, a toner image produced through a development process of supplying a liquid toner onto an image bearing body bearing an electrostatic latent image is transferred from the image bearing body onto an intermediate transfer body and then transferred from the intermediate transfer body onto a printing medium by use of a backup roller in a transfer-and-fixation zone. The system is characterized in that the intermediate transfer body and the backup roller are pressed against each other at a high pressure ranging from 10 kg/cm2 to 60 kg/cm2; no heating means is provided in the transfer-and-fixation zone; and the printing medium is preheated to a temperature required for transfer and fixation before the printing medium reaches the transfer-and-fixation zone.
In the transfer-and-fixation system for a liquid-development electrophotographic apparatus of the present invention, resin for use in the liquid toner has a softening temperature not higher than the withstand temperatures of members other than the intermediate transfer body such as a photosensitive drum, and the intermediate transfer body is provided with heating means for heating the intermediate transfer body to a temperature not lower than the softening temperature of the resin and not higher than the withstand temperatures of the other members. Also, the printing medium is preheated to a temperature required for transfer and fixation before the printing medium reaches the transfer-and-fixation zone.
Further, an intermediate transfer body suited for use in such a transfer-and-fixation system is characterized by including a tension textile layer which has undergone a stretching process effected in a direction of rotation of the intermediate transfer body, so as to enhance stiffness in expansion and contraction of the intermediate transfer body, and in that an image bearing layer is formed on the surface of the tension textile layer.